U.S. Pat. No. 4,330,836 to Donofrio, et al, assigned to Otis Elevator Company, explores techniques for measuring passenger load in an elevator. The patent comments that elevator cab load measurement is prone to inaccuracies from a number of factors, for instance, friction in devices that measure cab displacement under load and changes in the flexibility of the connecting pads that are typically positioned between the cab and load sensors (e.g. force transducers). It also focuses on variations in load measuring accuracy produced by passenger location (i.e. load distribution) in the elevator cab. The patent discloses a technique for locating force transducers strategically below the cab floor. The transducers measure cab load in a way that has been found to provide improved load weighing accuracy. A load line equation defines the cab load as a function of the aggregate of the transducer output signals. Passenger load, i.e. cab load, is then computed in a signal processor from the product of the aggregate and a gain coefficient; the product is then summed with an offset. The gain represents the slope of the line equation, the offset the value of the aggregate, theoretically zero, when the cab is empty.
A manual adjustment or calibration procedure to set the correct offset and gain is also explained in that patent. Potentiometers are adjusted to scale the aggregate of the transducer output signals to the actual load in the cab, ideally canceling out mechanically produced errors causing incorrect cab load measurement.
Another patent, also assigned to Otis Elevator Company, U.S. Pat. No. 4,305,495 to Bittar, et al, explores controlling elevator the dispatching interval between cars to satisfy hall calls and car call demands. The patent explains, among other things, a way to use the cab load as determined in U.S. Pat. No. 4,330,936 in a computer-based dispatching system--an elevator in which a high-speed signal processor, such as a microprocessor, rapidly performs a wide variety of computations based on the condition of the elevator cars, cab load being one condition. The processor produces signals manifesting those conditions and the signals are then used by the processor to control dispatching of each car from a landing. In this manner, the elevator performance is regulated and controlled in a scheme that provides optimal overall system performance. Among uses made of cab load, is motor torquing to hold the elevator car in place after the motor brake is lifted in preparation for acceleration away from a landing.
In another patent assigned to Otis Elevator Company, U.S. Pat. No. 4,299,309 to Bittar et al, a system for "an empty elevator car determination" is discussed. Activity of passenger-actuatable switches in the elevator cab, such as a car call buttons, open door button, the emergency stop switch and the like, is monitored as an indication of presence of passengers in the elevator cab. A preliminary determination is made that the car is empty if such activity is absent. If the condition exists for a particular period of time, the car is conclusively determined "actually empty".